Numbers and Quantities
Basic arithmetic operations
Number sets, arithmetic expressions and general arithmetic laws
What is a subtrahend, what does the distributive law mean, and what are rational numbers? - You can find explanations and exercises for these and other questions here.
Divisors and prime numbers
What a prime number is, what divisors are all about, and how to perform a prime factorisation - you can find explanations and exercises on such questions here.
Fractions and decimal numbers
Quantities and Units
Mass and length, area, volume, time and currencies - real-life applications usually deal with quantities and units and not just abstract numbers. You can find explanations and exercises here.